The auto industry has long sought to use lightweight materials as the base material for engine blocks so as to reduce overall vehicle weight and thus enhance fuel efficiency. Use of such materials (e.g., aluminum and its alloys) for automotive componentry such as engine heads and blocks often requires the incorporation of insert materials such as steel or cast iron to provide wear resistance which is not attainable with the lighter material on contact surfaces such as valve seats and cylinder bores. In particular, aluminum engine blocks require some type of wear-resistant surface on the cylinder bores, for example, to accommodate the sliding action of the piston sealing rings.
Previous approaches to this problem have been to use cast-in-place iron or steel liners in the cylinder bores, or to fashion the engine block out of materials such as the 390-type aluminum alloys, wherein a high fraction of primary silicon particles at the surface of the material provides the necessary wear resistance. Liners were found to be too heavy, limited heat conduction to the water jacket, required specialized facilities for either casting in place or inserting in the engine block, and were expensive to install. The use of 390 alloy for engine blocks, while solving the wear resistance problem, introduces other problems, such as difficulty in machining the material and specialized steps required to produce the most desirable wear surface.
Other approaches to the cylinder bore surfacing problem for aluminum alloys have used an electrodeposition process to produce layers which incorporate silicon carbide particles into a nickel matrix, such as the Nikasil process (registered trademark of the Mahle Company). The drawback of this technique is the complexity of the process for selective plating of engine cylinders requiring either localized deposition or extensive and elaborate masking. Wear-resistant coatings have also been deposited on engine parts using chemical vapor deposition (CVD) techniques, as disclosed in U.S. Pat. No. 5,226,975 (Denton et al.). These processes, however, can require 10 to 60 hours for a satisfactory coating to be deposited and thus are far too slow for assembly line purposes
Thermal spraying systems represent another approach to applying wear-resistant coatings to cylinder bore surfaces at processing rates significantly greater than other coating processes, such as CVD. These systems in general rely on a combination of heat and momentum to cause droplets of the coating material to conform and bond to the surface being coated. Different thermal spraying systems employ varied methods of imparting heat and momentum to a stream of droplets which will form the coating. One such system is the high velocity oxy-fuel (HVOF) process, such as disclosed in U.S. Pat. No. 5,019,429 (Moskowitz et al.). In the HVOF process, droplets attain a high velocity with high pressure gas as a transport medium and bond through plastic deformation upon impact with the coated surface. HVOF has been used for coating engine cylinder bores, as disclosed in U.S. Pat. No. 5,080,056 (Kramer et al.). The HVOF process, however, is slow (60 grams/minute), noisy (transport gases flow at supersonic speeds), and produces excessive heat which must often be dissipated from the workpieces by ancillary cooling schemes.
Another thermal spraying method, plasma spraying, uses a plasma arc to heat gases which heat and accelerate a droplet stream which is directed at a substrate rotating around a plasma torch by high pressure gas, as disclosed in U.S. Pat. No. 4,970,364 (Muller). Droplet velocities are lower than in the HVOF process but are heated to a higher temperature so that they are in a molten state upon reaching the substrate in order to provide a good bond. Other thermal atomization techniques, such as those used for powder production (Fabrication of Powders by the Rotating Electrode Process, Champagne and Angers, The International Journal of Powder Metallurgy & Powder Technology, Volume 16, No. 4, 1980), use a rotating rod of the feedstock material to impart momentum to the melted droplets. Powder production processes, however, are inadequate to form the required cylinder bore coating.